1. Field of the Invention
The present invention relates to a power amplifier module. More specifically, the present invention relates to technology effectively applicable to power amplifier modules for, e.g., cellular phones which need to keep high linearity under environmental changes such as changes of ambient temperature and control voltage.
2. Description of the Related Art
Portable telephones used in CDMA (Code Division Multiple Access), PDC (Personal Digital Cellular), TDMA (Time Division Multiple Access), and other systems are required to have high linearity and efficiency under environmental changes such as changes of ambient temperature and control voltage. To meet this requirement without allowing the performance of the telephone to drop, it is essential to maintain a stable operating point (idling current) of the power amplifier, which, among other components of the telephone has the greatest influence on the linearity and efficiency of the telephone, under environmental changes such as changes of ambient temperature and control voltage.
FIGS. 11A and 11B show examples of power amplifiers using GaAs-HBTs (Heterojunction Bipolar Transistors) presented in the C-10-7 of the 2000 General Conference of the Institute of Electronics, Information and Communication Engineers. FIG. 11A shows one of the unit amplifiers of a power amplifier. Reference numeral 1 is a power terminal; 14, a grounding terminal; 2, a control terminal; 8, an input terminal; 11, an output terminal; 22 to 24 and 28, GaAs-HBTs (hereinafter simply referred to as xe2x80x9ctransistorsxe2x80x9d); 21, 25, and 26, resistors; 27, a coupling capacitance; 29, an RF choke inductor; and 15, a bias circuit comprised of some of the above parts.
In the circuit of FIG. 11A, the transistors 24 and 28 connected by a Darlington connection and the transistors 22 and 23 connected by a diode connection constitute a current mirror circuit. If the ratio of the current mirror is set to xe2x80x9cnxe2x80x9d by setting the emitter area of the transistor 28 xe2x80x9cnxe2x80x9d times as large as the emitter area of the transistors 22 and 23, a current Iq, which is xe2x80x9cnxe2x80x9d times as large as a current Ib passing through the transistors 22 and 23 in the diode configuration, passes through the amplifying transistor 28 as an idling current. Keeping the current Ib constant is, therefore, important to stabilize the idling current Iq.
An input signal is input through the input terminal 8, amplified by the amplifying transistor 28, and output through the output terminal 11. A matching circuit (not shown) is connected to the input terminal 8; another matching circuit (not shown), to the output terminal 11. The resistor 26 and the RF choke inductor 29 are used to isolate signal components from the bias circuit 15 and the power line of low impedance.
In the circuit of FIG. 11B, an idling current Iq is stabilized by offsetting the temperature characteristics of base-emitter voltage of transistors 24 and 28 by the temperature characteristics of Schottky diodes 30 to 33 connected in series in bias circuit 15. Although the threshold voltage of the Schottky diode is about a half of that of the GaAs-HBT, the former""s temperature characteristics are almost the same as the temperature characteristics of base-emitter voltage of the latter. It is therefore possible to connect in series twice as many Schottky diodes as GaAs-HBTs, and any desired temperature compensation effect can be accomplished by changing the ratio of the resistance of resistor 21 to the resistance of resistor 36. In this unit amplifier, four Schottky diodes are connected in series to obtain temperature characteristics of the idling current Iq of a practical level. Reference numerals 34 and 35 also indicate Schottky diodes, which are dispensable from the point of view of the basic function of the unit amplifier.
The power amplifier of FIG. 11A is simple in configuration and has an advantage that all active elements can be formed by a transistor process. However, because a current Ireg passing through the resistor 21 is governed by: (i) the difference between the voltage at the control terminal 2 and the sum of the base-emitter voltage of the transistor 22 and that of the transistor 23; and (ii) the resistance of the resistor 21, the current Ireg varies in accordance with changes of ambient temperature and control voltage. The current Ib changes in proportion to the current Ireg. In other words, no means is provided for stabilizing the idling current Iq when changes of ambient temperature and control voltage occur. Therefore, the dependency of the idling current Iq on the ambient temperature and the control voltage is large. In addition, it is difficult to achieve a high yield because the performance of the unit amplifier is liable to be affected by the deviation in manufacture of resistors 21. When the control voltage is 2.8 V and the ambient temperature varies within a range of 30xc2x160xc2x0 C., the idling current varies within a range larger than xc2x140%. When the ambient temperature is 30xc2x0 C. and the control voltage lowers from 2.8 V to 2.7 V, the idling current Iq is reduced by 30% or so.
The idling current of the unit amplifier of FIG. 11B is stabilized by using Schottky diodes. This method, however, requires a Schottky diode process which is different from the GaAs-HBT process. Because it is difficult to stably control Schottky barriers and electrode contact in a Schottky diode process, it is difficult to manufacture power amplifiers at high repeatability and a high yield. As in the case of the unit amplifier of FIG. 11A, the unit amplifier of FIG. 11B has no means for stabilizing the idling current when a variation of the control voltage occurs. Accordingly, the idling current is reduced by 50% or so when the control voltage lowers from 2.8 V to 2.7 V.
In accordance with the above, an object of the present invention is to provide a power amplifier module with a bias circuit capable of feeding stable idling currents to power amplifier units. Another object of the present invention is to provide a power amplifier module which can be manufactured at low cost and a high yield by forming a bias circuit and power amplifier units in a stable transistor process.
A representative example of the invention disclosed in this application is as follows. A differential circuit provides error amplification with a first arrangement to detect changes of the control voltage and a second arrangement to detect changes of ambient temperature as mutual standard voltage inputs to produce an idling current. As a result, the effects of the changes of the control voltage and the ambient temperature are removed. The idling current controls the gain of a power-amplifying transistor. Input signals are fed to the power-amplifying transistor through a first matching circuit, and output signals from the power-amplifying transistor are fed to a load circuit through a second matching circuit.
Other and further objects, features and advantages of the invention will appear more fully from the following description.